2017 December 18

I have done a lot of EKG projects on this blog, mostly for the Applied Electronics course, where an EKG amplifier is Lab 12, but some just for fun. Today I decided to see whether one could do an EKG with just the Teensy LC board or just an Analog Discovery 2 USB oscilloscope.

The project was partly inspired by the Digilent post DIY ECG using a Analog Discovery 2 and LabVIEW, which I saw the title for and assumed that they were using just the USB oscilloscope. It turns out that wasn’t what they meant—it is just a simple 6-op-amp EKG amplifier looked at with the USB oscilloscope.

The setup I tried was about as simple as you can get—I put on three electrodes wired up as Lead I (see earlier post for this configuration), connected the body electrode to ground, and the other two electrodes to the plus and minus leads of channel 1 of the Analog Discovery 2.

The signal I got was quite small (about 1mV) and buried in 60Hz noise:

The raw signal from the oscilloscope, sampling at 1200 Hz, shows some spiking from the pulse, but a lot of noise. The big R spike is only about 3 LSB, so quantization noise is a problem.

I ran the recording through a digital filter to bandpass filter it to 0.1Hz–100Hz and put in a notch from 58Hz to 62Hz. The 100Hz low-pass had the effect of averaging out the noise, producing a signal with much finer resolution than the raw ADC values:

After filtering, the EKG signal is fairly clear. I don’t recommend trying to use only a couple of the lower-order bits of an ADC, but it is surprising how much information can be recovered by the filter.

I also tried using a Teensy LC board running PteroDAQ, using the A10–A11 differential channel. I had to bias my body between 0V and 3.3V, so I used a pair of 120kΩ resistors (one to GND, one to 3.3V) to connect to the body electrode.

Once again the raw signal was not great:

The signal had less noise than the signal to the Analog Discovery 2, but the signal was smaller also, negating the value of the finer steps of the ADC on the Teensy LC.

Once again, digital filtering restored the signal:

The signal-to-noise ratio here looks a little worse than for the Analog Discovery 2, despite the raw signal looking cleaner.

I managed to get a cleaner signal for the Analog Discovery 2 by turning off the surge protector, so that there was no 60Hz current anywhere nearby. The results after filtering were no better (and possibly worse) than from the signal with the 60Hz interference, so I did not bother plotting them for this post.

My conclusion is that it is possible to get EKG signals without adding an amplifier, but you can only see the signal clearly if you do some filtering. I’ll have to decide whether to recommend to students that they record signals directly from the EKG electrodes to get an idea what their amplifiers have to work with.

2014 June 29

Today I decided to solder the EKG design from Instrumentation amp from op amps fine for EKG onto one of my instrumentation amp protoboards, leaving out the instrumentation amp chip—I wanted to see how much trouble it would be. As it turned out, the build was fairly straightforward, but a little tedious. There are only dedicated spaces for 8 resistors on the board, but there are 9 resistors in the design I used, so one had to go elsewhere on the board. I deliberately left out the low-pass filter on this implementation (eliminating one capacitor), which did not make a huge difference—I ended up with about 58µV peak-to-peak of 60Hz noise on my input signal (compared to about 40µV in the previous design with a capacitor for low-pass filtering), which is fairly small compared to the 870µV R spike or the 220µV T wave. The 60Hz interference was large enough to interfere with the P wave and make it difficult to see whether or not there was a U wave. Of course, these measurements were made in my bedroom/lab, which has a lot less 60Hz interference than the lab the students work in. I’ll have to take the board into work and see how bad the interference is in that space.

Using a digital filter to remove the 60Hz noise reduced the 60Hz interference to under 100nV peak-to-peak (way lower than other noise components), producing very nice waveforms, even when sampling at 360 Hz. I’ll probably want to include a digital filter Python script in the book so that people can see the cleaned up signals, even if there isn’t room in the course to design digital filters.

I still have to decide whether to have students do the EKG amplifier without the INA126P chip, using only op amps. Wiring up the bigger circuit takes time, and I’m not sure that 6 hours of lab will be enough time for students to debug their design and get it soldered—it took them long enough to solder the EKG with the INA126P chip, which has fewer components and fewer wires to route. It took me quite a while to solder up the board, so it would probably take the students far too long. Is the pedagogic value of designing and building a 2-op-amp instrumentation amp worth the time? I do want the students to end up with an EKG to take home, as it is a tangible artifact that can demo the function of. I’m thinking that I could even drop the soldering of the pressure-sensor amp (since they don’t take home pressure sensors), and add soldering of the microphone pre-amp. If I do that, I’ll probably want to redesign the protoboard again, making it an op-amp protoboard with no instrumentation amp slot, but with more resistor spaces.

Cutting one part that costs about $2.70 and the $1.90 thermometer might justify my switching back to the resistor assortment I used in Winter 2013: 1120 piece resistor assortment for $17.39 instead of 1280 piece resistor assortment (currently $10.65) without raising the lab fee. Why would I want fewer resistors at a higher price? The 1120-piece assortment is 10 each of 112 values, while the 1280-piece assortment is 20 each of 64 values. Also the 64 values don’t seem to be very repeatable from set to set, and some sets has duplicates (so only 62 or 63 different values). The 112-value sets seem more reliably useful. A hobbyist might be better off going one step further to the 3700-piece resistor assortment (25 each of 148 values), but I can’t justify the $31.48 price for my class. (The extra $14 would probably raise the lab fee.)

2014 June 26

As I mentioned in Instrumentation amp from op amps still fails, I’ve been trying to decide whether to have students build an instrumentation amp out of op amps in the circuits course. I decided that it wouldn’t work for the pressure-sensor lab, because of the large DC offset. One could calibrate each amplifier, either in software (by recording a a few seconds of 0 pressure difference, and subtracting a constant fit to that region from the data) or in hardware, but I’d rather they had a more straightforward experience where the DC offset was small enough to be ignored.

I conjectured that instrumentation amp built from discrete op amps would work ok for the EKG lab, though, as the EKG already has to deal with much larger input voltage offsets due to differing electrode-skin contact. So I added a second stage with a gain of 81 to the instrumentation amp in the previous post with a gain of 19, to get a combined gain of 1539. I put in the high-pass filter needed to eliminate the DC offset, and a low-pass filter to reduce noise slightly (and make aliasing less of a problem). The corner frequency is a bit high (60Hz noise is not going to be reduced much), but that may allow a better view of the fast R spike in the EKG waveform.

The EKG circuit has four modules: a virtual ground (here set to 0.5v), an instrumentation amp, a high-pass filter to eliminate DC bias, and a second-stage non-inverting amplifier with some low-pass filtering.

The amplifier worked surprisingly well. I did sometimes have trouble with 60Hz noise, but it did not seem to be any worse than the amplifier based on the INA126P. I can remove the noise by digital filtering, though I’ve only played with that by post-processing the data files, not by designing a notch filter to run in realtime on the KL25Z (something to do when I have more time).

Here are a few traces made with EKG circuit above, feeding into the PTE20-PTE21 differential input on the KL25Z board, recorded using PteroDAQ.

This is lead I (LA–RA), without filtering, showing a rather disturbingly large 60Hz noise signal.

This is lead I (LA–RA), showing how the digital filter cleans up the signal. This was Bessel bandpass filtered to 0.3Hz to 100Hz, followed by notch 57Hz–63Hz, followed by notch 117Hz–123Hz. Each filter was a 5th-order Bessel filter, applied first forward in time then backward in time (using scipy’s filtfilt function).

This is lead II (LL–RA), which for some reason had rather low noise even without filtering.

I noticed that sampling at 360Hz allowed me to see a bit more of the structure of the S and T complex than I’ve seen previously, particularly in lead II, and I can even make out a little bump of a U wave just after the T wave.

I now have to decide whether to have students do the EKG amplifier without an INA126P chip, using only op amps. The design will be fairly heavily constrained, as they’ll need to get it all working on a single MCP6004 chip, but it will justify my spending a bit more time on how instrumentation amps work.

I may redesign the blinky EKG to use a single MCP6004 chip also, which would reduce the price of that substantially.

For the past couple of days I’ve been exploring variations on the Blinky EKG project, looking at alternative approaches.

For example, I looked at the possibility of eliminating the most expensive part (the instrumentation amp), and decided that building my own instrumentation amp out of op amps and discrete resistors was unlikely to be reliable. I discovered (after doing calculations for 2-op-amp and 3-op-amp designs) that 1% tolerance on the resistors would produce poor common-mode rejection. In Common-mode noise in EKG, I reported measurements of the common-mode noise with a fairly short twisted-pair connection to the EKG electrodes (close to a best-case scenario). I concluded that the common-mode noise was way too large for using unmatched resistors to be a reasonable design, and using an integrated instrumentation amp is still a good choice.

Yesterday, I tried turning the question around? Could I eliminate the op-amp chip? Currently, I’m using the op amp for two things: to provide a unity-gain buffer to make the reference voltage source between the power rails, and to do second-stage amplification after a high-pass filter removes the DC offset. To eliminate the op-amp chip, I need to replace both these functions.

Replacing the unity-gain buffer seems fairly easy—I could use a low-drop-out linear regulator to generate the reference voltage instead of a voltage divider and unity-gain buffer, which would be somewhat smaller and cheaper (11¢ rather than 25¢ in 100s). I didn’t have an LDO linear regulator at home, so I tried using a TL431ILP voltage reference instead. Unfortunately, it provides very little current, and was unable to maintain the desired voltage when hooked up to the reference voltage input of the instrumentation amp. I believe that a part like the LM317L would work fine, though, and I may want to test that at some point.

Removing the second-stage amplifier is more problematic. I can set the gain of the instrumentation amp up to 2000 or 2500 easily, but any DC component in the input differential signal results in saturating the amplifier (with a 6v output range, and a 1mV AC signal, we’d need the DC bias to be less than 1mV also to avoid hitting the power rails). I tried putting high-pass filters in front of the instrumentation amp, but with the long time constants needed to avoid filtering out the EKG signal, the filters never settled to within 1mV of each other, and the instrumentation amplifier always saturated.

So I need to keep the first-stage gain small enough to avoid saturation, which means I need a second-stage amplifier. I could use a single op amp for the second stage and a low-drop-out regulator for the reference voltage, which would produce a cleaner output signal (since my voltage-divider-plus-unity-gain-buffer reference introduces noise from the power lines). The MCP6001 single op amp is only 18¢ in 100s (rather than 25¢ for the MCP6002 dual op-amp), but the MCP6001 is only available as a surface-mount component, which I think is inappropriate for a first soldering project. The MCP6001 + LM317L would cost about 4¢ more than the MCP6002.

I considered redesigning the Blinky EKG to use the LM317L voltage regulator and the MCP6002, even though half the MCP6002 would be unused, but the LM317L needs a 1.5mA load to maintain regulation, and that seems like a lot for a battery operated device—more than the op amp or the instrumentation amp use (though less than the LED when it is lit). Even using a TL431ILPR voltage reference (10¢ in 100s) and the unity-gain buffer would only need 1mA, and would save one resistor. There are lower-current voltage references, like the LM385, but they cost a lot more (42¢ in 100s).

The non-rechargeable CR2032 batteries I’ve been using for the Blinky EKG have about a 225 mAh capacity, and cost about 19¢ in 100s (but the design needs 2, so 39¢). I could probably get about a 100-hour life with the present Blinky EKG design—I need to measure the current and the duty cycle of the LED to get a better estimate.

The Blinky EKG weighs about 20g (not counting the wires to the electrodes), which is a bit heavy for a pendant or brooch. Most of the weight is in the batteries, but a lighter battery would give up a lot of running time. The smaller batteries also cost a lot more, probably because Digikey only buys them in quantities around 10,000 rather than in the millions. (From other suppliers CR2032 batteries cost about 60¢ in 100s, not under 20¢).

It has been good to fool around with the Blinky EKG design, as it has gotten me to think a bit about design issues other than the first can-I-get-it-to-work one. I rarely get my students past that point in their thinking, and I’m not sure how I would do so, as there is always so much time pressure to cover new stuff, that they get very little time to tinker with designs.

I’ll have to make some measurements later this week to see how large the common-mode noise on the EKG signals really is. Of course, it is likely to be highly variable, depending on the electrodes and the wiring to them, but ball-park estimates would be useful.

If the AC common-mode voltage is ±200mV and we have worst-case resistor values, then we would have a ±8mV common-mode output from the instrumentation amp. With the lowest differential amplification (6.6 at Rgain=∞), a 1mV EKG signal would be smaller than the common-mode noise. Such a large common-mode voltage would easily justify the expense of the instrumentation amp chip. (Note: large DC common-mode voltages don’t matter, as the DC-blocking capacitor I used after the instrumentation amp can eliminate them.)

If the AC common-mode voltage is only ±1 mV, then the Blinky EKG could probably work even with very poor common-mode rejection in the instrumentation amp, and building it out of op amps and discrete resistors is feasible.

So today I tried measuring the common-mode signal. As it turns out, I had built an EKG amplifier on the protoboard that had a split Rgain resistor (two 12kΩ resistors in series). The node between the 2 resistors should be at (Vp+Vm)/2, the common-mode voltage. That node was fed to a unity-gain buffer, so I tried measuring the AC common-mode voltage.

I did this with two different tools: my Fluke 8060A multimeter (which measures RMS voltage) and my BitScope Pocket Analyzer, which can measure peak-to-peak voltage. With the multimeter, I saw about 20–35 mV RMS, and with the BitScope I saw 90–130 mV peak-to-peak (depending on where my arms were relative to the wires to the electrodes).

Trace of common mode noise recorded by the BitScope Pocket Analyzer, at 20mV/division. The fundamental frequency of the noise is 60Hz.Sorry about the black background—the manufacturers are promising a white background as an option in some future release of the software. Maybe they’ll add some labels for the x and y tick marks at the same time!

If I take the leads off the electrodes and clip them all together, then the common-mode noise drops below the level that the BitScope can measure (its noise floor is about 10mV). The multimeter also has trouble, reporting values from 0.05mV to 1.5mV. If I put a 3.3MΩ resistor between the two signal leads, and connect one of them to the Vref lead, then the multimeter reports about 13–20 mV RMS and the BitScope around 80–90 mV peak-to-peak.

The common-mode problems are definitely coming from the wiring to the electrodes. With 130mV peak-to-peak common-mode voltage and a common-mode gain of 0.02 or 0.04, we’d have 2.6–5.2 mV of common-mode noise at the output of an instrumentation amp made with discrete resistors and op amps. If the gain of the amplifier were around 6.6, we’d have a peak-to-peak signal of about 6.6 mV for the EKG signal, and the details of that signal would be buried in common-mode noise.

If we got lucky with the resistor values and they matched better than the specs claim (which is actually fairly likely), then the Blinky EKG would sort-of work with discrete op amps—as long as the 60Hz noise is no worse that what I saw. Many people are likely to be in electromagnetically noisier locations than my room, so the common-mode noise is likely to be much higher sometimes, and even with lucky matching of the resistors the common-mode signal is likely to swamp the real signal.

Thus I have confirmed that an integrated instrumentation amp is needed for the Blinky EKG, and there is no point trying to design one using discrete resistors and op amps.